A fuel cell utilizes energy generated at the time of a reaction combining hydrogen and oxygen. Fuel cells are expected to be installed and widely used as the next generation of electrical generating systems from the standpoints of energy conservation and the environment. There are a number of types of fuel cell, including solid electrolyte types, molten carbonate types, phosphoric acid types, and solid polymer types.
Among these types, solid polymer fuel cells have a high output density and can be made small, and they are easy to start and stop because they operate at a lower temperature than other types of fuel cells. Therefore, solid polymer fuel cells have attracted particular attention in recent years for use in small cogeneration systems in electric vehicles or for household use.
FIG. 1 shows the structure of a solid polymer fuel cell (referred to below simply as a fuel cell). FIG. 1(a) is an exploded view of a unit cell constituting a fuel cell, and FIG. 1(b) is a perspective view of an entire fuel cell formed by combining a large number of unit cells.
As shown in FIG. 1, a fuel cell 1 is a stack of unit cells. As shown in FIG. 1(a), a unit cell has a solid polymer electrolyte film 2, gas diffusion electrode layer 3 which functions as a negative electrode of the cell (also referred to as a fuel electrode film and referred to below as the anode) on one surface of the solid polymer electrolyte film 2, a gas diffusion electrode layer 4 which functions as a positive electrode of the cell (also referred to as an oxidant electrode film and referred to below as the cathode) on the other surface of the solid polymer electrolyte film 2, and separators (bipolar plates) 5a and 5b are stacked on both surfaces of the unit cell.
Some fuel cells are water-cooled fuel cells in which water-cooled separators having flow paths for cooling water are disposed between the above-described unit cells or between every several unit cells. The present invention also relates to a water-cooled fuel cell.
The solid polymer electrolyte film 2 (referred to below simply as an electrolyte film) comprises a fluorine-based proton-conducting film having a hydrogen ion (proton) exchange group. The anode 3 and the cathode 4 have a particulate platinum catalyst and graphite powder provided thereon, and if necessary, they may have a catalyst layer comprising a fluororesin having a hydrogen ion (proton) exchange group.
In this case, a reaction is promoted by contacting the catalyst layer with a fuel gas or an oxidizing gas.
Fuel gas A (hydrogen or a hydrogen-containing gas) is made to flow from passages 6a provided in the separator 5a to supply hydrogen to the fuel electrode film 3. An oxidizing gas B such as air is made to flow from passages 6b provided in separator 5b to supply oxygen. An electrochemical reaction is produced by the supplied gases to generate direct current electric power.
The primary functions demanded of a separator for a solid polymer fuel cell are as follows:
(1) a function as a passage uniformly supplying a fuel gas and an oxidizing gas to the interior surfaces of the cell,
(2) a function as a passage which efficiently discharges water produced on the cathode side and carrier gases such as air and oxygen after reaction from the fuel cell to the exterior,
(3) a function as an electrical connector which contacts the electrode films (anode 3 and cathode 4) and provides a conductive path between unit cells,
(4) a function as a partition between the anode chamber of one unit cell and the cathode chamber of the adjoining unit cell, and
(5) in a water-cooled fuel cell, a function as a partition between cooling water passages and the adjoining cell.
Materials for use as a substrate of a separator used in a solid polymer fuel cell (referred to below simply as a separator) which needs to perform these functions can be roughly divided into metallic materials and carbonaceous materials.
Separators made of metallic materials such as stainless steel, Ti, and carbon steel are manufactured by methods such as press forming. On the other hand, a plurality of methods are used for the manufacture of separators made of carbonaceous materials. Examples of such methods are a method in which a graphite substrate is impregnated with a thermosetting resin such as a phenolic or furan resin and cured and then baked, and a method in which carbon powder is kneaded with a phenolic resin, a furan resin, or tar pitch, the kneaded mixture is press formed or injected molded into the shape of a sheet, and the resulting material is baked and formed into vitreous carbon.
Metallic materials such as stainless steel have advantages such as the excellent workability which is characteristic of metals, as a result of which the thickness of a separator can be reduced, and a light-weight separator can be achieved. However, the electrical conductivity may be decreased due to elution of metal ions by corrosion or oxidation of the metal surface. Therefore, a separator made of a metallic material (referred to below as a metal separator) has the problem that the contact resistance between the separator and a gas diffusion electrode layer (referred to below for short as contact resistance) may increase.
On the other hand, carbonaceous materials have the advantage that a lightweight separator can be obtained. However, they had the problems that they were gas permeable and had low mechanical strength.
As one method of solving the above-described problems of metal separators, as disclosed in Patent Document 1, it has been proposed to perform gold plating on the surface of the substrate of a metal separator which contacts an electrode. However, using a large amount of gold in fuel cells for vehicles such as automobiles and stationary fuel cells is problematic from the standpoints of economy and quantitative restrictions on resources.
Therefore, it has been proposed to coat the surface of a metal separator with carbon as one attempt to solve the above-described problems without using gold.
The following techniques have been proposed as methods of covering the surface of a metal separator with carbon.
(A) A painted metal separator material for a solid polymer fuel cell disclosed in Patent Document 2 comprises an austenitic stainless steel member with a surface which has undergone pickling and an electrically conductive paint having a thickness of 3 to 20 micrometers on the pickled surface. The electrically conductive material inside the paint is a mixed powder of graphite powder and carbon black. That patent document discloses a process in which the surface of a substrate of a metal separator is pickled, and after pickling, the surface of the substrate is coated with an electrically conductive paint containing carbon.
(B) A paint for a fuel cell separator disclosed in Patent Document 3 uses graphite as an electrically conductive material. The paint is applied to the surface of a metal or carbon substrate of a separator for a fuel cell to form an electrically conductive coating. The paint contains a binder consisting of at least 10 percent by weight of a copolymer (VDF-HFP copolymer) of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), and an organic solvent which is miscible with the binder is used as a solvent. The ratio by weight of the electrically conductive material to the binder is 15:85 to 90:10, and the proportion of the organic solvent in the paint is 50 to 95 percent by weight.
Similar to Patent Document 3, Patent Document 8 discloses an electrically conductive separator in which an electrically conductive resin layer comprising a resin having a water repellant or basic group and electrically conductive particles is provided atop a metal substrate.
(C) Patent Document 4 discloses a separator for a fuel cell, the separator acting together with plate-shaped electrodes of unit cells to form a gas flow path. The separator comprises a metal sheet of low electrical resistance and an amorphous carbon film which covers the metal sheet and constitutes the surface of the gas flow path. The hydrogen content CH of the amorphous carbon film is 1 to 20 atomic percent. That document proposes a method of vapor deposition of a carbonaceous film using thin film-forming techniques (P-CVD, ion beam vapor deposition, or the like) instead of the above-described electrically conductive painted film.
(D) Patent Document 5 discloses a stainless steel sheet having a large number of minute pits formed over its entire surface, and a large number of fine projections are formed in the periphery of the pits. This stainless steel sheet is formed by immersing the stainless sheet in a ferric chloride solution and then carrying out alternating electrolytic etching.
Similar to Patent Document 5, Patent Document 7 discloses a separator plate having a surface coated with an oxidation resistant film. The surface is roughened to form irregularities. Portions where the coating is removed from the peaks of bumps become electrically conductive portions.
(E) Patent Document 6 discloses a means of heat treating a stainless steel member having carbonaceous particles adhered to its surface. A diffusion electrode layer is formed between the carbonaceous particles and the stainless steel. As a result, adhesion is increased, and electrical conduction between the carbon particles and the stainless steel can be achieved with certainty.